how to prevent arm from twisting with rig
Your issue is likely with blender's strange weighting system. Every vertex on a mesh is given a weight value (0-1, decimal based) of how much influence a particular bone has over that vertex. Your arm bone, of some farther down the arm, likely have weighting bleed over into the chest which is causing that twisting. Simply go into weight painting mode and adjust the weighting so anywhere not associated with the given bone is solid dark blue. This should take care of any unwanted twisting, unless you also have the same issue I do, where the armature seems to have applied it's self to the arms sideways, so they roll forwards and the elbow bends sideways. (I managed to get the IK and poll targets working too so this is really infuriating...)